The field of optical metamaterials has shown exciting advances in recent years, with many demonstrated applications based on their linear interaction with light, including super-resolution imaging and optical cloaking. More recently, optical metamaterials with a tailored nonlinear response have opened new degrees of freedom in metamaterial design, with interesting venues for super-resolution imaging, for performing efficient frequency conversion and optical control with greatly-relaxed phase-matching conditions as well as for optical switching and memories at the nanoscale.
So far, nonlinearities in metamaterials have been mostly realized by exploiting the natural nonlinear response of plasmonic metals or by enhancing the nonlinearity of optical crystals using plasmonic nanoantennas. A different approach to realize a large nonlinear optical response has been put forward by quantum-engineering intersubband transitions in n-doped multi-quantum-well (MQW) semiconductor heterostructures. By controlling the width of wells and barriers in the MQW structures, one can tailor the transition energy and dipole moments between electron subbands so as to maximize the quantum-mechanical expressions for a nonlinear process of choice thereby producing one of the largest known nonlinear responses, up to 6 orders of magnitude larger than that of traditional nonlinear optical materials. Voltage may be used to modify and spectrally tune intersubband nonlinearities and electrical pumping may be used to produce active intersubband structures with full loss-compensation for both second-order and third-order nonlinear processes.
Nonlinear MQW structures have been successfully integrated into waveguide-based systems to produce efficient frequency conversion and have enabled the development of mass-producible room-temperature electrically-pumped sources of THz radiation. Nearly 1% of Second Harmonic Generation (SHG) power conversion efficiency at 8.6 μm fundamental frequency was achieved in waveguides with passive In0.53Ga0.47As/Al0.48In0.52As MQW structures and over 16% power conversion efficiency was theoretically predicted. However, the integration of giant MQW nonlinearities with free-space optics is very challenging because optical transitions between electron subbands are intrinsically polarized along the surface normal to the MQW layers. As a result, traditional nonlinear intersubband MQW systems only interact with the light electric field polarization components normal to the MQW layers and the nonlinear response of the nonlinear intersubband MQW systems vanishes, for example, when input light is incident normal to the MQW layers (in this case, the electric field in the light wave is along the MQW layers).